Hey Kenny,
First thing I would check or replace is the radiator cap, these things are pretty old, (I have one), also make sure the air intake screen in front of the radiator is clean and the fins are clear, (a bit of air and or water will work here just not real high pressure), that screen in front of the rad is really bad about getting plugged up, (very fine screen).
As for the oil pressure if the engine gets too hot it will thin the oil and cause a drop in pressure, especially if it is close to time for an oil/filter change, or you don't have the right grade of oil.
These engines are also bad about scale buildup inside the water jackets, I just took the top plate off mine and got some pics of the buildup, I will post them tomorrow after I bring my camera home.
Also check both hoses to make sure they are not collapsing they don't have springs inside them and the bottom hose has more than a 90 degree bend in it!
Make sure you run coolant and distilled water in it, I usually run a bit more than 50/50 on mine, I lean to the heavy side with the coolant.
As for the radiator, mine was so heavy I knew it needed cleaned out, so I took it to a shop and theses days they want more than I paid for a new one, I am replacing the rad, both hoses, & cap, I am also putting an electric fan on mine, but that is because of other mods I have made, (added PS).
I am going to flush my engine by itself before I connect the new hoses and rad.
The cooling system is pretty simple on these,(no water pump), but everything has to be kept up to par or it will overheat.
If this is the first time it has done this, it should be fine, these engines are tough and dependable.
I know you aren't in the states, so I don't know if you can but the same brand of oil I use in diesels, but I will check the brand name tomorrow for you, it is a heavy duty oil made for diesels, it is all I run in all mine.
Cheers
Samantha